The evolution of metallocene-based catalysts for the polymerization of ethylene and higher .alpha.-olefins is reviewed in H. H. Brintzinger, et al., Angew. Chem. Int. Ed. Engl. 34: 1143-1170 (1995) and in P. C. Mohring, et al., J. Organometal. Chem. 479: 1-29 (1994). The applications of chiral metallocenes in organic synthesis are reviewed in R. L. Halterman, Chem. Rev. 92: 965-994 (1992). These reviews highlight the applications of state-of-the art metallocenes. Most often, these applications center on the use of titanium containing and other Group IV metallocenes.
The early preparations of Group IV metallocenes involved reactions of the metal tetrahalides, typically the tetrachlorides, with deprotonated ligands, such as sodium cyclopentadienide, to give the metallocenes in good yields. The metallocenes of current interest possess more complicated ligand structures, and their preparations are not as straightforward. For the preparation of these metallocenes, the use of titanium tetrachloride often results in low yields of the desired metallocenes. Titanium trichloride (TiCl.sub.3) is often specified for use in place of titanium tetrachloride (TiCl.sub.4); subsequent oxidation gives the desired metallocenes in greatly improved yields.
For some recent examples which specify the use of titanium trichloride, see L. A. Paquette, et al., Organometallics 14: 4865-4878 (1995); F. Zaegel, et al., Organometallics 14: 4576-4584 (1995); and M. E. Huttenloch, et al., Organometallics 11: 3600-3607 (1992). Halterman, supra, cites references which show the use of titanium trichloride in several metallocene preparations. The titanium trichloride so used is produced from commercial titanium tetrachloride. Titanium trichloride produced by hydrogen reduction of the tetrachloride is most often used in lab-scale preparations. For commercial-scale preparations, this is impractical due to cost and the presence of acidic impurities. These impurities require purification of the titanium trichloride, typically by preparation and isolation of an ether complex, usually the tetrahydrofuran complex.
Commercially-available titanium trichloride is produced by the reduction of the tetrachloride with alkyl aluminum compounds. The titanium trichloride so produced contains aluminum chloride, which is not removed. Typical analyses specify 76-79 weight percent of titanium trichloride with the remaining weight percent comprised mostly of aluminum chloride. The use of aluminum-reduced titanium trichloride in metallocene preparations often gives products which contain varying amounts of aluminum-containing impurities. Separation of these impurities from the product titanocenes is not straightforward in most cases, especially on a commercial scale. The presence of these impurities can have significant adverse effects during subsequent uses of the titanocenes, particularly in olefin polymerizations.
Accordingly, a need exists for a titanium trichloride reagent useful to produce titanocenes free of aluminum containing impurities.